The present invention relates to a ceramic sheathed-element glow plug for diesel engines.
A glow plug is described in German Published Patent Application No. 38 37 128 in which a ceramic heating device is held by the tip of a cylindrical holder. The ceramic heating device is electrically insulated with respect to the holder. Provided on the end of the cylindrical holder which is opposite to the ceramic heating device is a connector device which makes contact to the supply voltage. The ceramic heating device has a U-shaped heating segment, the two ends of the U-shaped heating segments each making contact with the connector device. During a preheat operation, a voltage is applied to the ceramic heating device so that a current flows from one end of the U-shaped heating segment via the tip of the heating segment on the combustion chamber side to the other end of the U-shaped heating segment. Due to the resistance of the ceramic, the current heats the heating segment so that the latter glows and the fuel/air mixture is heated for ignition.
The ceramic sheathed-element glow plug according to the present invention, having the features of the main claim, has the advantage that a very simple contacting of the ceramic heating element is possible without additional terminal contacts being sintered in. In addition, the contacting of the glow plug-shaped ceramic heating device without an adhering connection due to the simple design of contact surfaces in the insulation has the advantage that a sintered in metallic lead can be omitted. This ensures that the ceramic is not weakened by sintered-in foreign bodies, nor is the ceramic or the contact damaged during assembly. Finally, the production is simpler and accordingly more cost-effective. At the same time, the introduction of a packing between the internal housing wall and the external wall of the ceramic heating device brings about a very good seal in relation to the combustion chamber with simultaneously improved contacting.
It is particularly advantageous that the glow plug has a segment of greater diameter in the area remote from the combustion chamber, since the shoulder thus produced between the segment with smaller diameter and the segment with greater diameter compresses the gasket material when the glow plug is inserted into the housing and thus ensures a very good seal. The seal of the components of the sheathed-element glow plug that are remote from the combustion chamber against the combustion chamber is considerably improved. Finally, the surface pressures for attaining a reliable contact with ground and engine compartment sealing are minimized, which in turn reduces the tangential tensile stresses in the ceramic sheathed-element glow plug. Furthermore, the use of a contact spring for the connection of the face of the first end of the U-shaped heating device, which is remote from the combustion chamber, in order to make contact with the supply voltage makes it possible to compensate for varying layer thicknesses between the insulating layer and the recesses in this insulating layer. This also ensures that a reliable contact is made. Since the pressure forces for achieving a reliable contact are low in the glow plug according to the present invention, no additional tensile stresses are produced in the ceramic glow plug. The use of a powder seal as the sealing compound between the housing and the glow plug ensures that no special requirements are placed on the surface quality of the seal. As a result, no excess stresses can arise due to surface roughness.